In practice, dinosaur fossils are defined by the presence of certain features in the bones of the skull, shoulder, “hand”, rear leg and hip. Dinosaurs developed an upright gait, rather than the sprawling gait of other reptiles (where the legs stick out to the sides of the body) still observed in lizards and crocodiles today.

Scientists look for common skeletal traits shared by the common ancestor of all dinosaurs, and its descendents, a group they call Dinosauria. Dinosaurs are distinct from other prehistoric groups of flying, swimming or terrestrial reptiles, such as pterosaurs, ichthyosaurs or the sail-backed Dimetrodon.

All dinosaurs belong to one of two groups that diverged very early in their evolution. One is the Ornithischia (or bird-hipped dinosaurs), which include triceratops, stegosaurus, and the hadrosaurs or duck-billed dinosaurs. The other is the Saurischia (or lizard-hipped dinosaurs), which includes two-legged predators such as Velociraptor and Tyrannosaurus, and the massive plant-eating titanosaurs and brachiosaurs.

Most palaeontologists believe that birds evolved from small, two-legged predatory dinosaurs, and some even call them “avian dinosaurs”. In case new discoveries change the set of traits used to classify dinosaurs, palaeontologists formally define Dinosauria as all animals descended from the last common ancestor of birds and triceratops.

They usually describe an important feature of the fossil, its discoverer, or the place where it was found. Scientists attempt to translate the names into Greek. Many include the suffix “-saur” which means lizard or reptile in Greek. For example, Dinosauria means “terrible lizard”, a name describing the size of the first dinosaurs recognised in the 19th century. Triceratops means “three-horned face”. Herrerasaurus is named after its discoverer, Victorino Herrera.

Like other plants and animals, dinosaurs have a genus name, which is capitalised, and a species name, which is not. For example, Tyrannosaurus – which means tyrant lizard – is a genus name, and rex is the name of the largest species of tyrannosaur. Formal rules cover naming, and dictate that the first given legitimately shall be permanent, so scientists now choose names very carefully.

According to dinosaur diversity expert Steven Wang at Swarthmore College in Pennsylvania, US, there are approximately 550 widely accepted genera of dinosaurs (representing about 650 to 700 species), and new ones continue to be discovered. This figure is likely to be just a fraction of their true diversity, as many species may be absent from the fossil record.

Using statistics Wang and palaeobiologist co-worker Peter Dodson, estimate that as many as 1,900 dinosaur species may have existed in total, meaning just a fraction have so far been discovered in the fossil record.

Some names have turned out to be mistakes. For example, Brontosaurus and Apatosaurus, initially thought to be different genera, were later merged. Under the formal naming rules, Apatosaurus – the first legitimate name given – became the official name.

Scientists group dinosaurs according to their view of the evolutionary family tree. Three major groups evolved from Ornithischia&colon; the armoured dinosaurs such as ankylosaurus, the horned and frilled dinosaurs such as Triceratops, and the two-legged hadrosaurs; all were plant-eaters. The best-known saurischians were the two-legged predators called theropods, and the massive four-legged plant-eaters called sauropods.

They form when plant or animal remains are covered by sediment, which then turns into stone. The best-known fossils are bones, which absorb minerals from the sediment and turn into rock that differs from the surrounding matrix. Sometimes a dead animal is buried between layers of rock, which form an impression or cast that can be split apart along the layer containing the bones, producing a part and counterpart. In a few cases, the casts include impressions of feathers or skin that decayed long ago.

Footprints can also become fossilised if the mud where they form is buried and turns to rock. Fossilisation requires burial by accumulating sediment, so it is much more likely in the sea, a stream, or a lake than on dry land.

The first dinosaurs lived about 230 million years ago. They ran on their hind legs, with their forward-tilted body balanced by their tail. Ranging from the size of a coyote to a large kangaroo, many were probably predators, but some evolved quickly into plant-eaters. The oldest known dinosaurs were unnamed plant-eaters which lived in Madagascar about 230 million years ago and slightly younger meat-eaters called herrerasaurs from South America. The oldest South American dinosaur ever found is a herrerasaur called Eoraptor.

Artists’ impressions of dinosaurs are based on the available evidence. This comes from fossilised bones, footprints, and a handful of skin and feather impressions. Palaeontologists and artists use their knowledge of the anatomy of living animals and fossil imprints of muscles to visualise a living animal with flesh on the bones. In some cases, they have evidence for skin texture, or the presence of feathers or feather-like body coverings. In others, they assume that dinosaurs had skins like living reptiles such as crocodiles or lizards.

Artists may give dinosaurs colours like modern crocodiles, reptiles, birds or mammals, but it is nearly impossible to know what colours dinosaurs really were.

Andrew Parker, an evolutionary biologist at the University of Oxford, UK, has nevertheless had limited success in reconstructing the colour of ancient fish, crustaceans and beetles. He examined tiny colour-producing structures, or types of pigment cell, embedded in well-preserved fossils. Dinosaurs – particularly feather-bearing specimens – could plausibly be a future project for Parker, especially as modern bird’s iridescent shades are produced by tiny structures that might, occasionally, survive in fossils.

The smallest adult dinosaur known is a hummingbird. But excluding modern birds, the smallest adult dinosaur known was unearthed from China’s 124 million-year-old Yixian formation in 2000. That two-legged predator, named Microraptor, was just 39 centimetres long. But only 15 centimetres of that length was taken up by the body, the rest by a long straight tail. Some baby dinosaurs in eggs were smaller, and smaller adults probably existed too, but tiny bones are rarely preserved.

The largest dinosaurs were long-necked plant-eaters called sauropods. Brachiosaurus is the largest one whose remains are reasonably complete. Its skeleton was up to 23 metres long, and its shoulders were about 12 metres above the ground. Scientists have found fragments of bigger sauropods – the biggest of these seems to be Argentinosaurus, which may have been about 40 metres long and weighed 100 tonnes.

Sound cannot be fossilised, but we do have a few clues. A group of hadrosaurs called lambeosaurs had large hollow bony crests on their skulls. The hollow tubes formed a convoluted nasal passage, which grew as the animal matured. The twisted tubes in the skull of Parasaurolophus forced air to go through a passage up to 2 metres long in adults, and a model of its skull makes sounds like a wind instrument. Living dinosaurs probably did not sound quite like this, because the noise would have been affected by soft tissue that has not fossilised. Scientists believe many other dinosaurs would have been vocal, as are their closest living relatives, the birds.

Large animals benefit from the economies of scale. Their bodies require less energy per kilogram of mass, and their sheer size makes adults difficult to attack. Being tall also allows them to eat food out of the reach of smaller animals. The ratio of adult to newborn size was much greater among dinosaurs than mammals. We do not know why, but some palaeontologists think that dinosaurs may have continued growing throughout their adult lives, so the largest dinosaurs were also the oldest.

Large size may also have allowed dinosaurs to maintain a high body temperature, without the need to generate heat metabolically as mammals and birds do today. Australian zoologists have demonstrated that in crocodiles – today’s heftiest living reptiles – large individuals are consistently warmer, and show less fluctuation in body temperature, than smaller ones. Though large and warm dinosaurs would have enjoyed the speed and endurance associated with birds and mammals today, large dinosaurs in warm climates may have faced a considerable problem of overheating.

It is extremely unlikely that enough dinosaur DNA could be recovered from fossils to create a viable dinosaur using cloning techniques. DNA is a very fragile molecule that starts breaking down soon after death. Scientists have recovered DNA and proteins from bones that have been preserved for hundreds of thousands of years in cold areas, but only small fragments of genes remain. Dinosaur fossils are thousands of times older, and cloning would require the complete genome, not to mention a suitable host egg.

Comparing the genomes of living birds might give us a better insight into the DNA of their dinosaur ancestors, but would not directly allow us to reconstruct the genes that dinosaurs lost when they evolved into birds. In 2002, researchers reconstructed the eye pigment protein of an archosaur – a crocodile-like dinosaur ancestor that died out 240 million years ago – by examining and comparing the DNA of its modern relatives, including crocodiles, birds and fish.